Update the logic for repetable strings.

[KevinJump]

Fixes logic for when a repeatable string is
"inside a nested content item - inside a nested content item - inside a doctype grid editor - inside a Grid."

• this does change how we seariaze nested content slightly (it returns the JToken to the upper level not an object of string).

Testing required before merge!!!

I think we returned a string here because a) it worked and b) it might have had a knock on effect to Block List or Contentment Blocks. - so before release we will need to confirm they don't get effected , although i think they now have override methods in place so don't use this base method for the final extraction of their string.